1. Field of the Invention
This invention relates to a method for recovering oil from a subterranean, viscous oil-containing formation by injecting a slug of CO.sub.2 carbonated water containing a CO.sub.2 solubility promoter to increase the amount of CO.sub.2 injected into the formation, injecting a slug of a CO.sub.2 solubility demoter into the formation to decrease the solubility of the CO.sub.2 in the aqueous slug thereby increasing the amount of CO.sub.2 available for reducing oil viscosity and injecting water to displace the mobilized oil toward a production well for recovery.
2. Background of the Invention
A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. These techniques include thermal recovery methods, waterflooding and miscible flooding.
Fluid drive displacement of oil from an oil-containing formation utilizing CO.sub.2 is known to have the following effect in enhancing the recovery of viscous oils: (1) oil swelling, (2) viscosity reduction and (3) when dissolved in an aqueous driving fluid, it dissolves part of the formation rock to increase permeability. As the oil viscosity increases, a straightforward CO.sub.2 immiscible flood becomes less effective because of gravity override and viscous fingering due to unfavorable mobility ratio as disclosed in the article by T. M. Doscher et al, "High Pressure Model Study of Oil Recovery by Carbon Dioxide", SPE Paper 9787, California Regional Meeting, Mar. 25-27, 1981. In such cases, carbonated waterflood has been found to be more effective, both in the laboratory tests and through computer simulation studies as set forth in a paper by L. W. Holm, "CO.sub.2 Requirements in CO.sub.2 Slug and Carbonated Water Oil and Recovery Processes", Producers Monthly, September 1963, p. 6, and a paper by M. A. Klins et al, "Heavy Oil Production by Carbon Dioxide Injection", CIM Paper 81-32-42, 83rd Annual Meeting, Calgary, May 3-6, 1981. However, the solubility of CO.sub.2 in water is quite limited and it decreases with the salinity of the water as disclosed in the aforementioned article by L. W. Holm. As a result, carbonated-water projects generally were not successful because of the insufficient transfer of CO.sub.2 from water to oil.
The present invention provides a method for increasing the amount of CO.sub.2 available in the formation to enhance recovery of oil by first increasing the solubility of CO.sub.2 in carbonated water injected into the formation and subsequently injecting a CO.sub.2 solubility demoter to release CO.sub.2 from the injected carbonated water.